The present invention relates to a method for separating impurities from the machine circulation of the mixture circulating through the coating station, of coating stations used in the coating and sizing of paper and board.
The invention also relates to an arrangement intended to apply the method.
At a coating station or a sizing station, coating is spread by an application device on the paper or board web to be coated and the coating layer is leveled to the set thickness using a doctor device. The application device usually spreads a large excess amount of coating and in many application devices a so-called return flow is used, which does not adhere to the web. The return flow of the application device and the excess doctored from the web are returned to the coating machine reservoir, from where the coating is pumped to the coating station. The coating pumped to the coating station must be clean, so that there is a pressure sieve in the machine circulation between the feed pump and the station, by means of which impurities brought with the web, pieces of paper that have entered the circulation from a web break, and possible lumps of hardened coating that have formed are removed from the coating. The sieves used are mainly washable sieves operating on the back-wash principle, or sieves made by coiling a triangular wire, which are kept clean by a mechanical doctor. Back-wash sieves are being withdrawn from use, because a considerable amount of waste water with a coating content arises in the washing of the sieve, which loads the mill's water-treatment system. The raw-material losses of washable sieves are greater than those of mechanically cleaned sieves.
Mechanically cleaned sieves are closed pressurized vessels, in which the coating travels through the sieving element from a higher pressure to a lower pressure. The sieving element can be either a perforated plate, or a cylinder wound from a single triangular wire, in which a gap for penetration is formed between the wire windings. The gap width of the wire drum is about 75-150 μm and its length is very great, because a single gap forms a sieving surface over the entire length of the wire. The sieve is cleaned by moving a doctor element over the surface of the sieve cylinder and the reject that collects in the sieve is removed by opening the reject valve at intervals of, for example, 8 hours. From the reject valve, a few tens of liters (10-30 l) of coating is released and led into the mill's drainage system. Although such pressure sieves have a large sieving capacity relative to the sieving surface area, the size of the gap means that they cannot effectively separate long fibrous impurities. Fibrous impurities collect in the machine circulation of the coating stations and, when they become caught under the coating station's doctor element, cause streaks in the coating and thus spoil the paper being manufactured. Another drawback caused by fibrous impurities is blockage of the sieve gap when elongated fibers become caught in it. The elongated fibers become oriented parallel to the flow, so that they either so through the gap or become jammed in it. Long fibers enter the machine circulation from the coating return circulation and are formed of fibers that have detached from the web being coated and of synthetic fibers that have detached to the surface of the web from the drying felt, and which in turn detach from the web during coating. These impurities cause most of the need to service and clean the sieve, so that it would be important to remove them from the machine circulation. A drawback with mechanically cleaned pressure sieves is still the large amount of reject coating that loads the cleaning devices, because usually there are several sieves in a coating line and tens of liters of reject must be removed from each one about three times each day.
If perforated plates are used as the sieving elements, the fibrous impurities can be removed quite well. The drawback with these sieves is, however, a considerably lower capacity than gap sieves of a corresponding hole size. If the width of the gap of a gap sieve is 150 μm, it can be used to achieve a sieving capacity of about 40 l/s. Using a plate sieve with a corresponding hole seize, a capacity of about 15 l/s can be achieved. Thus at large flows more perforated-plate sieves will be required, or else they can be used to process only some of the circulating flow.
A drawback in sieving carried out on the high-pressure side of the coating station is still the fact that, because the coating-agent reservoir, which is below the machine level, of the coating station or sizing station is considerably lower than the station itself, the pressure of the mixture must be raised considerably. Thus the pressure over the sieve may be as much as 2-6 bar, whereas it only needs to be 0.1-0.2 bar over the sieve to operate. This means that on the feed side of the coating station a high-efficiency feed pump is required, as well as piping, sieves, and air bleeds that are designed to withstand a high pressure. When operating at a high pressure, the soft impurities collapse and may then pass through the sieve drum and even threaten to break the sieve drum. On the other hand, the return pipe of the coating station is often partly empty, because it is dimensioned for the largest flow used and there are no devices that cause flow resistance, prior to the machine reservoir. When the pipe is empty, coating agent may be precipitated in it, which must be cleaned or which may start moving into the machine circulation.
In order to remove fibers that have entered the machine circulation, the mixture returning from the coating station have been at least partly sieved with oscillating sieves. However, oscillating sieves have such a limited capacity that the full return flow cannot be sieved without increasing the number of oscillating sieves unreasonably, or by reducing their separation ability by using sieving elements with larger openings. Oscillating sieve also have the drawback that a large amount of air penetrates the coating mixture that falls under the sieve fabric as drops. This form of sieving is mainly used to sieve the small amount of mixture that circulates during web breaks.